Sintered ferrite magnets are used in various applications such as motors, electric generators, speakers, etc. As typical sintered ferrite magnets, Sr ferrite (SrFe12O19) and Ba ferrite (BaFe12O19) having a hexagonal M-type magnetoplumbite structure are known. These sintered ferrite magnets are relatively inexpensively produced by a powder metallurgy method using iron oxide, carbonate of strontium (Sr) or barium (Ba), etc. as raw materials.
To provide electric parts of automobiles, parts of electric equipments, etc. with reduced size and weight and higher efficiency for environmental protection, etc., sintered ferrite magnets are recently required to have higher performance. Particularly demanded in motors used in electric parts of automobiles are sintered ferrite magnets having such high coercivity HcJ that they are not demagnetized by a strong demagnetizing field even when made thinner, while keeping high residual magnetic flux densities Br.
To provide sintered ferrite magnets with improved magnetic properties, JP 10-149910 A and JP 11-154604 A propose methods for improving HcJ and Br by substituting part of Sr with rare earth elements such as La, etc. and part of Fe with Co in the above Sr ferrites.
Sr ferrites having part of Sr substituted by rare earth elements such as La, etc., and part of Fe substituted by Co, etc. (hereinafter referring to as “SrLaCo ferrite”), which are described in JP 10-149910 A and JP 11-154604 A, have excellent magnetic properties, so that they are widely used in various applications in place of conventional Sr ferrites and Ba ferrites. However, further improvement of magnetic properties is desired.
As sintered ferrite magnets, Ca ferrites are also known in addition to the above Sr ferrites and Ba ferrites. It is known that Ca ferrites have a stable structure expressed by the composition formula of CaO—Fe2O3 or CaO-2Fe2O3, and that La added provides the ferrites with a hexagonal crystal structure. However, they have magnetic properties on the same level as those of conventional Ba ferrites, not sufficiently high.
Japanese Patent 3181559 discloses a Ca ferrite having part of Ca substituted by rare earth elements such as La, etc., and part of Fe substituted by Co, etc. for improved Br and HcJ, and improved temperature characteristics of HcJ, by having an anisotropic magnetic field HA of 20 kOe or more (hereinafter referring to “CaLaCo ferrite”). It describes that this anisotropic magnetic field HA is 10% or more higher than that of Sr ferrites.
However, CaLaCo ferrites have Br and HcJ on the same level as those of SrLaCo ferrites and extremely poor Hk/HcJ, despite a high anisotropic magnetic field HA, failing to meet both requirements of high HcJ and high Hk/HcJ, so that they have not been used yet in various applications such as motors, etc.
To improve the magnetic properties of CaLaCo ferrites, various proposals have been made. For example, JP 2006-104050 A proposes a CaLaCo ferrite having optimized atomic ratios of constituent elements and an optimized molar ratio n, with La and Co at a particular ratio. WO 2007/060757 A proposes a CaLaCo ferrite having part of Ca substituted by La and Ba. WO 2007/077811 A proposes a CaLaCo ferrite having part of Ca substituted by La and Sr.
WO 2008/105449 A proposes a method of reducing the particle sizes of crystal grains to increase a magnet density, and controlling the shapes of crystal grains to improve magnetic properties, in a composition containing more Sr and/or Ba than in WO 2007/060757 A and WO 2007/077811 A, by a pulverization process comprising a first fine pulverization step, a heat-treating step of powder obtained in the first fine pulverization step, and a second fine pulverization step of repulverizing the heat-treated powder (hereinafter referring to “heat-treating and repulverizing step”).
However, the CaLaCo ferrites described in JP 2006-104050 A, WO 2007/060757 A, WO 2007/077811 A and WO 2008/105449 A need about 0.3 of Co by atomic ratio, despite improved magnetic properties than those of the CaLaCo ferrite proposed by Japanese Patent 3181559, so that more Co should be used than in sintered SrLaCo ferrite magnets commercially available presently (containing about 0.2 of Co by atomic ratio). Co is ten to several tens of times as expensive as iron oxide, a main component of ferrite magnets. Cost increase of raw materials is thus unavoidable, resulting in more expensive sintered ferrite magnets. Particularly, WO 2008/105449 A conducting a heat-treating and repulverizing step cannot avoid cost increase because of increase in production steps, suffering increase in both raw material cost and production cost. Accordingly, it fails to satisfy a market demand of cost.
Because the biggest advantage of sintered ferrite magnets is inexpensiveness, even sintered ferrite magnets having high magnetic properties would not be accepted in the market if they were expensive.
If the Co content were reduced to less than 0.3 by atomic ratio in CaLaCo ferrites, their magnetic properties would be on the same level as those of SrLaCo ferrites, losing the advantages of the CaLaCo ferrites having a high anisotropic magnetic field HA.